Early on in their study, researchers took DNA from the orangutans, which showed them to be "peculiar" compared to other orangutans in Sumatra.

So the scientists embarked on a painstaking investigation - reconstructing the animals' evolutionary history through their genetic code.

One of the lead researchers, Prof Michael KrÃ¼tzen from the University of Zurich, Switzerland, explained to BBC News: "The genomic analysis really allows us to look in detail at the history.

"We can probe deep back in time and ask, 'when did these populations split off?'."

The analysis of a total 37 complete orangutan genomes - the code for the biological make-up of each animal - has now shown that these apes separated from their Bornean relatives less than 700,000 years ago - a snip in evolutionary time.

Head to head

For his part in the study, Prof Serge Wich, from Liverpool John Moores University, focused on the orangutans' signature calls - loud sounds the male apes make to announce their presence.

"Those calls can carry a kilometre through the forest," Prof Wich explained.

"If you look at these calls, you can tease them apart, and we found some subtle differences between these and other populations."

The final piece of the puzzle, though, was very subtle but consistent differences in the shape of the Sumatran, Bornean and Tapanuli orangutan skulls.

Prof Wich told BBC News that the decades of collaborative genetic, anatomical and acoustic studies had achieved an "amazing breakthrough".

"There are only seven great ape species - not including us," he said. "So adding one to that very small list is spectacular.

"It's something I think many biologists dream of."

New and disappearing

But this newly described great ape will be added to the list of Critically Endangered species, just as it is added to the zoological textbooks.

"It's very worrying," said Prof Wich, "to discover something new and then immediately also realise that we have to focus all of our efforts before we lose it."